SLPVectorize: Put horizontal reductions feeding a store under separate flag

Put them under a separate flag for experimentation. They are more likely to
interfere with loop vectorization which happens later in the pass pipeline.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191371 91177308-0d34-0410-b5e6-96231b3b80d8